A chemical, immunological or biological sensor instrument is selective for a particular target analyte. Such a sensor can, and often does, contain an immobilized chemical sensing species. The chemical sensing species is an organic, inorganic or biological recognition element that selectively recognizes a target analyte. In order for the sensor to operate, the analyte and the immobilized species must be able to bind together to produce an optical, mass, or electrochemical response related to the concentration of the desired analytes. The selection of particular chemical sensing species for a targeted analyte is well known.
What is not well known, is a method to effectively immobilize the chemical sensing species while maintaining its affinity and intrinsic function over time. There are various techniques to immobilize the species. These techniques can be classified generally as covalent and non-covalent attachment techniques. The covalent technique is initially attractive, but is often tedious, labor intensive, and requires relatively expensive reagents or environmentally unattractive solvents. Further, covalent attachment is dependent on specific functionalities being present on the species such that it can be attached to surface moieties. In other aspects of covalent bonded sensors the recognition chemistry is generally limited to monolayer coverages.
The non-covalent techniques, in contrast, capitalize on entrapping or adsorbing the species to form the sensor. One such technique is a sol-gel process. Sol-gel derived composites serve to entrap the chemical sensing species within a macroscopic solid. Unfortunately, sol-gel based sensors that use monoliths are not practical for most real-world measurements because of the inherently long response times associated with the long, circuitous diffusion pathlength of the analyte and the limited accessibility of an entrapped chemical sensing species to the analyte. In order to exploit the advantages of sol-gels and improve the response times and overall sensor performance, diffusion pathlengths must be decreased and/or the accessibility of the sol-gel entrapped chemical sensing species increased.
Thin sol-gel films, in principle, can solve these pathlength and accessibility matters. Common methods to prepare sol-gel derived thin films include dip and spin casting. These methods entail loading the chemical sensing species directly into the sol solution. Conceptually this is not a problem. Realistically, if the chemical sensing species is expensive or difficult to obtain, this method is wasteful and less attractive. L. L. Hench and J. K. West, Chem Rev., 90 (1990) 33. This method also creates a rough and cracked sol-gel film.
Another method to prepare thin layer films is disclosed in U.S. Pat. No. 3,840,391, by Spitz et al. This method entails preparing a metal-ligand composition in an aerosol generating instrument. The instrument comprises an ultrasonic wave emitter which is placed on the underside of an annular tank containing an ultrasonic-wave transmitting liquid. The instrument also comprises a diaphragm that closes-off an atomization chamber that is constituted by a cylindrical tube provided at the top with a conical end portion in which is inserted a head. The head carries a duct through which the aerosols are discharged. The aerosoled film is then applied upon a heated substrate having a temperature of at least 450.degree. C. In most instances, the heated substrate is a glass plate.
A problem to solve is to provide a sol-gel based sensor that effectively immobilizes a chemical sensing species while maintaining the species' affinity and intrinsic function over time.
Another problem to solve is to provide a method to effectively immobilize the chemical sensing species while maintaining its affinity and intrinsic function over time within a sol-gel film.
Another problem to solve is to provide a new aerosol generation instrument for the deposition of sol-gel derived thin films.
Another problem to solve is to create a relatively smooth and crack-free sol-gel film.
In yet another problem to solve is to control the porosity of the thin sol-gel layer.
Another problem to solve is to control the hydrophobicity or hydrophilicity characteristics of the sol-gel layer.
Another problem to solve is applying a thin sol-gel layer upon a substrate at ambient temperatures to ensure the integrity, stability and functionality of the entrapped chemical sensing species.